composites and advanced materials in aircraft

The
Lockheed F-22 uses composites for at least a third of its structure.

For many years, aircraft designers could
propose theoretical designs that they could not build because the
materials needed to construct them did not exist. (The term "unobtainium"
is sometimes used to identify materials that are desired but not yet
available.) For instance, large spaceplanes like the Space Shuttle would
have proven extremely difficult, if not impossible, to build without
heat-resistant ceramic tiles to protect them during re-entry. And
high-speed forward-swept-wing airplanes like Grumman's experimental X-29
or the Russian Sukhoi S-27 Berkut would not have been possible without the
development of composite materials to keep their wings from bending out of
shape.

Composites are the most important
materials to be adapted for aviation since the use of aluminium in the
1920s. Composites are materials that are combinations of two or more
organic or inorganic components. One material serves as a "matrix," which
is the material that holds everything together, while the other material
serves as a reinforcement, in the form of fibres embedded in the matrix.
Until recently, the most common matrix materials were "thermosetting"
materials such as epoxy, bismaleimide, or polyimide. The reinforcing
materials can be glass fibre, boron fibre, carbon fibre, or other more
exotic mixtures.

Fiberglas is the most common composite
material, and consists of glass fibres embedded in a resin matrix.
Fiberglas was first used widely in the 1950s for boats and automobiles,
and today most cars have fibreglass bumpers covering a steel frame.
Fiberglas was first used in the Boeing 707 passenger jet in the 1950s,
where it comprised about two percent of the structure. By the 1960s, other
composite materials became available, in particular boron fibre and
graphite, embedded in epoxy resins. The U.S. Air Force and U.S. Navy began
research into using these materials for aircraft control surfaces like
ailerons and rudders. The first major military production use of boron
fiber was for the horizontal stabilizers on the Navy's F-14 Tomcat
interceptor. By 1981, the British Aerospace-McDonnell Douglas AV-8B
Harrier flew with over 25 percent of its structure made of composite
materials.

Making composite structures is more
complex than manufacturing most metal structures. To make a composite
structure, the composite material, in tape or fabric form, is laid out and
put in a mould under heat and pressure. The resin matrix material flows and
when the heat is removed, it solidifies. It can be formed into various
shapes. In some cases, the fibres are wound tightly to increase strength.
One useful feature of composites is that they can be layered, with the
fibres in each layer running in a different direction. This allows
materials engineers to design structures that behave in certain ways. For
instance, they can design a structure that will bend in one direction, but
not another. The designers of the Grumman X-29 experimental plane used
this attribute of composite materials to design forward-swept wings that
did not bend up at the tips like metal wings of the same shape would have
bent in flight.

The greatest value of composite
materials is that they can be both lightweight and strong. The heavier an
aircraft weighs, the more fuel it burns, so reducing weight is important
to aeronautical engineers.

Despite their strength and low weight,
composites have not been a miracle solution for aircraft structures.
Composites are hard to inspect for flaws. Some of them absorb moisture.
Most importantly, they can be expensive, primarily because they are labour
intensive and often require complex and expensive fabrication machines.
Aluminium, by contrast, is easy to manufacture and repair. Anyone who has
ever gotten into a minor car accident has learned that dented metal can be
hammered back into shape, but a crunched fibreglass bumper has to be
completely replaced. The same is true for many composite materials used in
aviation.

Modern airliners use significant amounts
of composites to achieve lighter weight. About ten percent of the
structural weight of the Boeing 777, for instance, is composite material.
Modern military aircraft, such as the F-22, use composites for at least a
third of their structures, and some experts have predicted that future
military aircraft will be more than two-thirds composite materials. But
for now, military aircraft use substantially greater percentages of
composite materials than commercial passenger aircraft primarily because
of the different ways that commercial and military aircraft are
maintained.

Aluminium is a very tolerant material and
can take a great deal of punishment before it fails. It can be dented or
punctured and still hold together. Composites are not like this. If they
are damaged, they require immediate repair, which is difficult and
expensive. An airplane made entirely from aluminium can be repaired almost
anywhere. This is not the case for composite materials, particularly as
they use different and more exotic materials. Because of this, composites
will probably always be used more in military aircraft, which are
constantly being maintained, than in commercial aircraft, which have to
require less maintenance.

Thermoplastics are a relatively new
material that is replacing thermosets as the matrix material for
composites. They hold much promise for aviation applications. One of their
big advantages is that they are easy to produce. They are also more
durable and tougher than thermosets, particularly for light impacts, such
as when a wrench dropped on a wing accidentally. The wrench could easily
crack a thermoset material but would bounce off a thermoplastic composite
material.

In addition to composites, other
advanced materials are under development for aviation. During the 1980s,
many aircraft designers became enthusiastic about ceramics, which seemed
particularly promising for lightweight jet engines, because they could
tolerate hotter temperatures than conventional metals. But their
brittleness and difficulty to manufacture were major drawbacks, and
research on ceramics for many aviation applications decreased by the
1990s.

many modern light aircraft are constructed in composite
material such as this Glasair

Aluminium still remains a remarkably
useful material for aircraft structures and metallurgists have worked hard
to develop better aluminium alloys (a mixture of aluminium and other
materials). In particular, aluminium-lithium is the most successful of
these alloys. It is approximately ten percent lighter than standard
aluminium. Beginning in the later 1990s it was used for the Space Shuttle's
large External Tank in order to reduce weight and enable the shuttle to
carry more payload. Its adoption by commercial aircraft manufacturers has
been slower, however, due to the expense of lithium and the greater
difficulty of using aluminium-lithium (in particular, it requires much care
during welding). But it is likely that aluminium-lithium will eventually
become a widely used material for both commercial and military aircraft.

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